This invention relates to an apparatus for coating a traveling web of paper, which is frequently, but not always, done on a papermaking machine. More particularly, this invention relates to an apparatus for coating a traveling web utilizing a multi-chamber short dwell coater having a flow separator plate defining at least one orifice through which a coating liquid can flow during the coating process to reduce localized cross-machine pressure variances, reduce overall coating liquid pressure on the web, and improve coating quality regardless of web travel speed.
A multi-chamber short dwell coater in accordance with the present invention applies coating on a web at high machine speed, and has a metering zone separated from an application zone and an outflow zone. The metering zone is separated from the application zone by a flow separator to reduce propagation of flow instabilities into the metering zone. The outflow zone is extended by a baffle plate to increase the size of the contact zone and reduce air bubble entrainment in the coating. The result is improved uniformity and runnability of coatings at high machine speed.
Slot coaters have been used in low speed coating of photographic film and other coating processes. A major limitation to increasing the application speed of slot coaters is the inability to match the coating fluid velocity with that of the moving web. In addition, slot coaters require a vacuum to stabilize the contact line between the coating material and the web which limits the speed at which coating can be applied.
Coating apparatus for applying a liquid coating onto a traveling web having a partitioned coating chamber are disclosed in Suzumura et al., U.S. Pat. No. 5,173,120, having, for example, chambers A and B separated by a partition plate 10. A pre-metering blade 11 prevents air bubble entrainment and isolates areas of fluid instability created in an outflow region from the metering zone. A potential problem associated with this coater design is increased frequency of paper breaks resulting from the use of multiple blades coming in contact with the web.
The width of chamber B in the machine direction is such that the coating liquid can circulate within this chamber to form undesirable vortices. The vortices are unavoidable with such a design because chamber B must be relatively wide to accommodate the converging and flexible pre-metering blade 11 attached to the upper distal end of the separator partition 10 that extends downstream to converge with the web 3 as it passes over the backing roll 2. Further, coating liquid flowing over the top of the relatively short dam plate 8 can be entrained with air traveling on the surface of the paper web at the converging gap between the blade 11 and the traveling web. Entrained air diminishes coating quality and increases streaking problems.
The pre-metering blade 11 is described as being resilient (col. 3, line 28), which can become unstable during operation and adversely affect the pressure of the coating liquid as it is supplied to the surface of the traveling web. Pressure variations such as these result in non-uniformity and lower coating quality. With such pressure variations, the blade 11 can be misaligned or even "chatter" during operation and eventually its purpose is defeated.
Further, the full force exerted by the coating liquid against the web in the Suzumura et al. coater is produced by the liquid pressure over the machine-direction length of the pond at the top of chamber A and chamber B. Force over the converging blade 11 is a function of the integral of the pressure profile over the effective length of the surface of the converging blade. The integration produces a relatively large force value. Significantly, the effective blade length in the machine-direction is relatively long, so the integration produces a relatively large force value regardless of the pressure of the coating liquid. This high force can result in the web being impregnated with coating liquid as opposed to applying coating liquid to the surface for a better finished paper quality.
Thus, it is desirable to have a coater that provides uniform liquid coating pressure with low forces and minimized air entrainment to provide high quality coatings at high machine speeds.